1. Field of the Invention
This invention relates to fabrication technology used in the assembly of integrated circuit packages in the microcircuit manufacturing industries, with particular attention to lead frame design.
2. Description of the Prior Art
In standard industrial practice often the lead frame of an integrated circuit (IC) assembly is shaped so that the upper planar surface of the die attach pad is parallel to and slightly below the planar surface of the lead frame. This alignment or offset is usually achieved by "downsetting" the tie bars during the formation of the lead frame. This vertical offset provides a smaller overall cross sectional area for the encapsulated die and die attach pad in the finished integrated circuit package. The smaller area contributes to smaller package size, increased circuit density and often greater physical stability for the assembled package.
Unfortunately, however, fabrication experience has shown, in zig zag inline packages (ZIP) using only tie bars or having the two main tie bars normal to the third tie bar, that tie bars and die attach pads often bow or become distorted during the die attach and encapsulation processes resulting in improper spatial relationship of the die attach pad relative to the package causing excessive package bowing. This distortion causes mechanical and electrical failure within the integrated circuit packages and results in loss of system integrity and quality. Experience has also shown that lead frames having larger die attach pads, more lead fingers and longer tie bars seem more sensitive to physical distortion during encapsulation and trim and form steps and thus are more likely to produce more imperfect IC product per unit operation.
Typical configuration of the lead fingers in a conventional multiple lead zig zag inline (ZIP) lead frame is shown in FIG. 1 and will be discussed in detail later. It consists generally of a flat rectangular shaped die attach paddle which is held on the opposing ends of one longer side and on the same axis by tie bars which extend laterally outwardly and in the same plane as the die attach pad. These tie bars are attached on their opposing ends to respective opposing parallel sides of the lead frame and are downset therefrom in a conventional manner to the plane of the die attach pad as described. Conventional industry practice does not include the exposure of tie bars on the top of a ZIP configuration because of possible electrical shorting problems.
The die attach pad for a ZIP configuration typically also has an additional tie bar which extends normally outwardly and in the same plane from a point midway in the opposing longer side of the die attach pad, as shown in FIG. 2. This tie bar generally also is downset in a conventional manner to the plane of the die attach pad from the plane of the multiple lead fingers which usually are in the same plane and in parallel spaced relationship with the tie bars.
The intention of this invention is to provide a new and improved method of ZIP IC package fabrication which will reduce the failures due to bowing inherent in the present technology and insure greater IC unit integrity uniformity and strength.